The purpose of a drop bolt is to provide a concealed lock for a door or a movable barrier. In prior art devices a solenoid moves a bolt axially out from the doorway frame and into a strike plate set in the edge of the door. The strike plate has an aperture dimensioned to axially receive the bolt so that once it is in place the door is effectively locked. Such devices are generally used where security is desired as well as concealment of the installation when the door is locked.
There are however a number of problems with the devices currently in use. If there is any misalignment of the door in the frame the bolt will hit the strike plate rather than seat in the hole. While with some systems the bolt will pulse for a limited time the door will still only lock if the strike plate hole is precisely aligned with the bolt. There are also further problems associated with releasing a locked door. For example the lock can jam if any side loads are imposed from air pressure differentials, persons pressing against the door or door warpage etc. In such cases the bolt becomes jammed in the hole and cannot be withdrawn by the relatively weak pull of the solenoid. The power required by a solenoid system to slide the axially moving bolt into and out of the locking aperture even during normal unhindered operation is also excessive. While the applicant's co-pending Australian patent application 66599/00 discloses a more direct and power efficient drive for such locks comprising a screw shaft rotatable by an electric motor to move a roller nut along said shaft and engage a cam surface on a pivotal bolt this mechanism still has disadvantages. In order to provide for fail safe operation the bolt is spring biased to the withdrawn position and the drive mechanism is mounted in a cage which is also pivoted in the lock housing to disengage the nut from the cam surface. Such arrangement requires excessive depth in the housing to accommodate the pivoting movement of the cage. Also, during disengagement of the roller nut from the bolt there is a possibility of the drive mechanism becoming unsynchronized and jamming.